The invention relates generally to magnetic disk drives and, more particularly, to a disk drive with a sampled servo control system with a feed-forward control path that outputs a feed-forward control. The result is a closer approximation to a desired seek profile that provides a quieter faster seek over short to medium seek lengths.
Magnetic disk drives generally read and write data on the surface of a rotating magnetic disk with a transducer that is located at the far end of a moveable actuator. A servo control system uses servo control information recorded amongst the data, or on a separate disk, to controllably move the actuator from track to track (xe2x80x9cseekingxe2x80x9d) and to hold the actuator at a desired position (xe2x80x9ctrack followingxe2x80x9d). A detailed discussion of servo control systems is unnecessary because such systems are well known as set forth, for example, in patent application Ser. No. 09/138,841 now U.S. Pat. No. 6,204,988 that was filed on Aug. 24, 1998, entitled xe2x80x9cDISK DRIVE CAPABLE OF AUTONOMOUSLY EVALUATING AND ADAPTING THE FREQUENCY RESPONSE OF ITS SERVO CONTROL SYSTEM,xe2x80x9d and is commonly owned by the assignee of this application.
Most disk drives have previously been used for storing conventional data files of the type that are associated with personal computers. In such applications, data integrity is paramount relative to other considerations such as seek times and the reduction of acoustic noise. Disk drives, however, are becoming popular for recording and replaying audiovisual dataxe2x80x94e.g. a drive based recording device that replaces a video cassette recorder (VCR). A drive-based recording device of this nature will benefit from using a disk drive with faster seek times because it will spend less time moving its actuator where it needs to be and more time recording or recovering information such that it may be able to record and/or playback more audiovisual data streams than otherwise possible. At the same time, a drive-based recording device is likely to be located adjacent to a television or be in some other location where acoustic noise is undesirable. Accordingly, it is equally important for the disk drive to implement its seeks as quietly as possible.
The drive industry has progressed through several stages of development as related to seeks. Of relevance here, is the prior use of a so-called xe2x80x9cbang bangxe2x80x9d seek profile wherein the transducer is rapidly accelerated at the start of a seek and then rapidly decelerated at the end of a seek. A bang-bang seek profile moves the transducer to a target position in as rapid a manner as possible. On the other hand, since the bang-bang profile is a square wave, it contains many high frequency components that may overlap with and detrimentally excite a mechanical resonance that causes the transducer to take longer to settle into the target position. It has previously been determined that a quieter, more efficient seek is possible by xe2x80x9cshapingxe2x80x9d the transducer""s acceleration profile so that it does not appear like a square wave, but rather approximates a single frequency sine wave. The result is a shaped seek profile that is xe2x80x9cclosexe2x80x9d to a bang-bang square wave that does not contain the high frequency components that may excite the drive""s resonant frequencies.
Modern disk drives, however, generally use a sampled servo control system that only periodically receives position information (e.g. once per servo sector) and shortly thereafter outputs a corrective feedback command effort signal based on a deviation between the indicated position and the target position. Accordingly, the drive""s ability to provide a shaped acceleration profile during a given seek is limited by the fact that such drive must make a piecewise approximation of that profile with a finite number of command efforts. The more servo sectors encountered during the seek, the more command efforts and the better the approximation. The longer the seek, the better the approximation as well. Accordingly, the sinusoidal seek profile is approximated quite well on longer seeks (e.g. 1,000 tracks or more), but tends to appear quite unlike a sine wave for medium seeks (e.g. 256 to 1,000 tracks) and for short seeks (e.g. 1 to 255 tracks). A xe2x80x9cfull strokexe2x80x9d seek length in a modern drive is in the order of 10,000 tracks such that the short to medium strokes referenced herein are substantially less than a full stroke. A short seek that might be accomplished in only four or five samples, for example, tends to warp the sinusoid into something more akin to a triangle wave that is more abrupt, acoustically louder, and contains more energy in the higher frequency components that may excite a resonance and extend settling time. As a result, conventional disk drives tend to take longer than necessary to settle and be louder than necessary during short to medium seeks.
There remains a need, therefore, for a disk drive that implements short to medium seek lengths with less settling time and less acoustic noise, i.e. is faster and quieter and more suitable, therefore, for audiovisual applications.
In a first aspect, the invention resides in a disk drive comprising a plant having a transducer that periodically samples servo position information recorded on the disk at a servo sampling rate to produce an indicated position signal, and a voice coil motor adapted for moving the transducer in response to a total command effort signal; and a servo controller that generates the total command effort signal for moving the transducer from a start position to a target position. In this first aspect, the servo controller comprises a reference position generator that provides a reference position signal having a continuous rate of change for moving the transducer from the start position to the target position in a controlled fashion; a feedback control path that receives the reference position signal and the indicated position signal and applies a feedback command effort signal at the servo sampling rate based on a difference between the reference position signal and the indicated position signal; and a feed-forward control path that includes a multi-rate state machine and receives the reference position signal and applies a feed-forward command effort signal at a feed-forward rate that is greater than the servo sampling rate to move the transducer more closely along the shaped position.profile between servo samples.
In a second aspect, the invention resides in a disk drive comprising a plant having a transducer that periodically samples servo position information recorded on the disk at a servo sampling rate to produce an indicated position signal, and a voice coil motor adapted for moving the transducer in response to a total command effort signal; and a servo controller that generates the total command effort signal for moving the transducer from a start position to a target position. In this second aspect, the servo controller comprises a reference position generator that provides a reference position signal having a continuous rate of change for moving the transducer from the start position to the target position in a controlled fashion; and a feed-forward control path that includes a multi-rate state machine and receives the reference position signal and applies a feed-forward command effort signal at a first feed-forward rate that is greater than the servo sampling rate to move the transducer more closely along the shaped motion profile between servo samples for a first seek length that is less than a predetermined seek length and at a second feed-forward rate that is equal to the servo sampling rate for a second seek length that is longer than the predetermined seek length.
In a second aspect, the invention resides in a disk drive comprising a microprocessor for executing servo and non-servo programs; a disk having recorded servo position information; a transducer for periodically sampling the recorded servo position information at a servo sampling rate to produce an indicated position signal; and a servo controller that generates a total command effort signal for moving the transducer from a start position to a target position. In accordance with this aspect of the invention, the servo controller comprises a reference position generator that provide a reference position signal that varies as a function of time along a shaped position profile for moving the transducer from the start position to the target position in a controlled fashion; a feedback control path that receives the reference position signal and the indicated position signal and applies a feedback command effort signal at the servo sampling rate based on a difference between the reference position signal and the indicated position signal; and a feed-forward state machine that receives the reference position signal and concurrent with the execution of non-servo programs by the microprocessor, autonomously applies a feed-forward command effort signal at a feed-forward rate that is greater than the servo sampling rate to move the transducer more closely along the shaped position profile between servo samples.